Support for the Giant Impact Theory
Ebb and Flow also spotted many large, linear structures under the moon's surface that can run for up to 300 miles (480 km). These subsurface "dikes" of solidified magma are covered by craters, suggesting that they predate most of the moon's violent impacts.
The dikes could only have formed if the moon's crust were extending, making room for the magma, researchers said. This would happen if the moon's interior were heating up and expanding, as predicted by the leading theory for the moon's origin - the Giant Impact hypothesis.
This idea posits that a Mars-size body smashed into Earth about 4.5 billion years ago, and the moon coalesced from pieces of our planet that were blasted into space.
"The process of building a moon out of that debris should result in a situation where the moon is cooler on the inside and warmer on the outside," said Grail guest scientist Jeff Andrews-Hanna of the Colorado School of Mines. "And then what naturally happens is that the interior will warm up and expand during that first billion years."
"This had been predicted theoretically a long time ago, but there was no direct observational evidence to support this period of early lunar expansion until this Grail data," Andrews-Hanna added.
Martian Life Deep Underground?
Grail's revelations about the violent early history of the inner solar system suggest that the upper crusts of its rocky planets are highly and deeply fractured, Zuber said.
Such cracks could provide a pathway for fluids, perhaps explaining what happened to the ocean that some scientists think existed long ago on the surface of Mars.
"That ocean could well be underground," Zuber said.
As the Martian surface dried out over the years, such undergound water may have provided a refuge for surface microbes, if they ever existed. Microbes "could have gone very deep within the crust of Mars," Zuber said.
Ebb and Flow wrapped up their primary science mission in May and are currently embarked on an extended mission that will end in mid-December. Shortly thereafter, they will be crashed intentionally onto the lunar surface; exactly where and when that will happen is still being worked out, researchers said.
The results presented today were from the primary mission. The spacecraft have been circling even closer to the moon during the extended phase - an average altitude of 14 miles (23 km) as opposed to 34 miles (55 km) - so the Grail team thinks the gravity map will get even better.
"We expect a lot more exciting results in the future, so stay tuned," said Grail project scientist and co-investigator Sami Asmar of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
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Holy GRAIL! Photos From NASA's Moon Gravity Mission 20 Most Marvelous Moon Missions Lumpy Gravity Has The Moon – GRAIL to Learn Luna Images: The lunar gravity field as measured by NASA's Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012. Very precise microwave measurements between two spacecraft, named Ebb and Flow, were used to map gravity with high precision and high spatial resolution. Image credit: NASA/JPL-Caltech/MIT/GSFC Copyright 2012 SPACE.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.